This application relates generally to photoconductor crystal growth, and in particular to image acquisition devise and x-ray detectors.
Digital x-ray imagers are progressively replacing films in many medical and scientific applications. These imagers generally use a pixelated array of thin film transistor (TFT) switches fabricated from amorphous silicon (a-Si) deposited on a flat glass substrate. The switches define addressable pixels. At each pixel, there is a photodiode and a capacitor connected to the TFT. A layer of scintillator material covers the array of photodiodes. The scintillator is typically a compound such as gadolinium oxysulphide or cesium iodide that emits light when it absorbs x-ray photons. When x-ray photons enter the scintillator layer, they are absorbed and visible light is emitted. A proportion of the emitted light illuminates the photodiode located at the pixel beneath the point where the x-ray photons are absorbed. The light causes current to flow through the photodiode, which deposits charge in the capacitor. For readout the TFTs are switched on in sequence via gate lines, and the integrated charge collected on each capacitor is read out via orthogonal data lines to an ND converter. The amount of charge collected per pixel is proportional to the local x-ray flux absorbed in the scintillator, and the digitized values of charge per pixel form a digital image of the spatial distribution of x-ray flux incident onto the detector.
Conventional x-ray detectors employ indirect detection of x-ray photons. The energy of the photons is first converted into visible light and the visible light photons subsequently illuminate a photodiode, resulting in the collection of electric charge. In conventional x-ray detectors, some optical photons never reach the photodiodes beneath the scintillator layer and their energy is lost, which reduces the sensitivity of the imager to x-rays. Some photons are emitted in directions such that they illuminate nearby photodiodes rather than the photodiode at the pixel where the x-ray photons are absorbed. This causes loss of image resolution.